The present invention relates to an exhaust gas purifier for eliminating particulate matter contained in exhaust gas of an engine which is installed in a car, an industrial machine or the like, and which uses petroleum fuel as energy.
An engine using petroleum fuel as energy burns the fuel to convert it into mechanical energy. However, exhaust gas discharged from the engine partially contains particulate matter (hereinafter abbreviated to "PM") mainly containing carbon because of incomplete combustion. If such PM is discharged as it is, air pollution is caused.
Various techniques for eliminating PM in such exhaust gas, particularly PM in exhaust gas of a Diesel engine, have been introduced.
However, exhaust gas discharged from an engine is high in temperature, and contains corrosive gas such as SOx. Accordingly, the material of a filter has been difficult to select. In addition, PM in the exhaust gas includes very small particles. Accordingly, attention has been paid also to the fineness of the mesh of the filter.
Typically, ceramic foam of cordierite has been used as a material, and this ceramic foam is formed into a honey-comb shape. This ceramic foam material has such a fine mesh as to trap PM more securely and advantageously than other materials having a mesh which is not so fine. However, this material needs to have more effective area for trapping PM because its fineness of mesh is rather disadvantageous with respect to the quantities of PM to be trapped. Therefore, it is necessary to form the material into a honey-comb shape such that a more effective area for trapping PM will be obtained. With respect to this material, there is another disadvantageous problem that will be observed when the trapped PM is burned and regenerated. If this material is heated locally by such burning and regeneration of PM, cracks or melting loss are likely to occur.
Recently, such series of porous metal as Cr--Al, Ni--Cr--Al, or Fe--Ni--Al have been developed to be use for the materials of filters. These materials have an advantage in that they do not cause local heating which is a weak point of the above-mentioned cordierite and regeneration can be attained by uniform heating. However, when their mesh is made as fine as the ceramic foam, a very heavy filter is produced because of a difference in specific gravity between the materials. Therefore, various contrivances have been utilized for the structure of a filter.
As for the structure of a filter, Japanese Patent Unexamined Publication No. Hei-6-257422 discloses a structure in which two or four piled cylindrical filter elements respectively made from three-dimensional mesh-structure porous metal are used, and a heater is installed between the filter elements. Advantageously, this filter can burn and regenerate trapped PM very effectively and uniformly, and, in addition, has a long life. However, the filter does not have such a fine mesh as the ceramic foam because of the characteristic of its material. In addition, in structure, exhaust gas passes the respective filter elements only once. Accordingly, the performance to trap PM in the exhaust gas is not satisfactory.
Japanese Utility Model Unexamined Publication No. Hei-1-66418 discloses another method in which a plurality of cylindrical filter elements are combined, and the roughness of meshes of the filter elements are made different from each other to thereby eliminate PM in exhaust gas effectively, though nothing is referred to about material to be used.
In this method, the filter element on the exhaust gas inlet side is designed to have a rough mesh and a large area to trap, while the filter element on the exhaust gas outlet side is designed to have a fine mesh and a small area to trap. In such a manner, large particles in the exhaust gas are eliminated by the former, that is, the rough-mesh filter element, while small particles in the exhaust gas are eliminated by the latter, the fine-mesh filter element.
Though this is a good idea, the PM trapping quantities of the respective filter elements are different from each other in accordance with the size of the PM particles in the exhaust gas. As a result, the performance of pressure loss of the filter due to the exhaust gas is dominated by one of the filters which is inferior in the performance of pressure loss.
In addition, in order to eliminate PM of smaller particle size, it is necessary to make the meshes of the latter filter element finer. The performance of pressure loss is dominated by this fine-mesh filter element on the outlet side. Further, since the above-mentioned prior art does not disclose any means for regeneration, it is not practical in use.
As has been described above, a purifier for engine exhaust gas may be produced for practical use, but it still leaves room for improvement.
Particularly, it is expected to provide a long-life purifier which can continuously and repeatedly perform trapping of PM in exhaust gas and burning/regenerating of the trapped PM in a state where the purifier is left as it is attached to a discharge passage of the exhaust gas.
To provide such a purifier, the performance to trap PM in exhaust gas must be improved, but, at the same time, the performance of the regeneration must be improved because it largely affects the life of the purifier.
A purifier which can balance these performances with improved efficiency is required. First, with respect to the material to be used, in order to make regeneration stable and make the life of the purifier long, material, such as ceramics, having a low coefficient of thermal conductivity is not suitable. In the case of metal material, it is necessary to increase volume porosity in order to solve the problem of large specific gravity. However, if the volume porosity is made too large, the trapping performance is lowered, and, at the same time, the size of the purifier is increased.
On the contrary, if the volume porosity is made small, the pressure loss of the filter due to exhaust gas is increased. In this case, it is necessary to increase the trapping area correspondingly, so that the purifier increases in size. However, if a purifier is made large in its size for the above reason, it would be inconvenient to use such a purifier for purifying the exhaust gas emitted from an engine since the space for mounting the purifier on a vehicle is so limited.